I read about our Prophets (saw) uncle who was very good to him and who died without accepting Islam. What I don't understand is that why didn't rasool saw pray to Allah to make him accept islam?

We should understand that whatever happens occurs through the will and decision of Allah Ta'ala. Allah Ta'ala in His infinite knowledge knows the reasons and wisdom behind every decision. Nevertheless, the incident of the uncle of Nabi (Sallallaahu Alayhi Wasallam), Abu Talib is narrated as follows:

When Abu Talib was about to pass away, Nabi (Sallallaahu Alayhi Wasallam) came to him. Abu Jahl and Abdullah ibn Abi Umayyah ibn Mugheera were also present. Nabi (Sallallaahu Alayhi Wasallam) then said to him, 'O My Uncle, say Laa ilaaha illallaah', I will intercede through it on your behalf in the court of Allah.' Abu Jahl and Abdullah ibn Abi Umayyah immediately remarked, 'O Abu Talib! Are you going to turn away from the religion of Abdul-Muttalib?' but Rasulullah (Sallallaahu Alayhi Wasallam) continued inviting him and presenting to him the same words, until the last words that came out from the lips of Abu Talib was that he was on the religion of Abdul-Muttalib and he refused to say 'Laa ilaaha illallaah'.

Nabi (Sallallaahu Alayhi Wasallam) said, 'I will continue to make Du' aa-e-Maghfirat for you as long as I am not stopped from it.' Thereafter, Allah Ta'ala revealed the verse '. It is not allowed for a Nabi or the believers to make Du'aa-e-Maghfirat for the Mushrikeen even though they may be close relatives.' And Allah Ta'ala revealed especially to Nabi (Sallallaahu Alayhi Wasallam) regarding Abu Talib, '. Indeed you cannot guide who you wish bur Allah Ta'ala guides whomsoever he wishes.' (Qasas Aayat56) - Tafseer ibn Katheer part3 pg.408.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai